Carefully remove roast to a cutting board. Strain liquid
from slow cooker into a bowl, reserving onions. Cut roast
horizontally in half, then crosswise into slices. In a small bowl, blend mayonnaise and horseradish. Spread 1 1/2 tbsp mayo blend on each roll. Top with beef and some of the reserved onions. Serve each sandwich with 1/3 cup of the liquid (jus) for dipping.